As one of measures to improve a system throughput in a cellular network, there is a technique of arranging a plurality of small base-station devices in a macrocell provided by a base station device. Because an area of a small cell provided by the small base-station device is small, a mobile terminal device is not necessarily present in the small cell. Accordingly, in a state in which no mobile terminal device is present in the small cell, electric power consumed by the small base-station device is wasted. Thus, in 3GPP (3rd Generation Partnership Project), Energy Saving function is proposed as one of SON (Self Organization Networks) functions (NPTL1).
A small base-station device having the Energy Saving function has an active state and an inactive state. In the active state, such device performs a normal operation as the small base-station device. In the inactive state, power saving of the entire network is realized by stopping radio transmission in a part or the whole of the cells. The state-transition of the active state/inactive state of the cell can be controlled according to a traffic amount. For example, based on statistical data of traffic change, the number of the small base-station devices in operation is increased in a time zone, such as a traffic peak time, in which the traffic amount is large. In a time zone, such as an off-peak time, in which the traffic amount is small, the number of the small base-station devices in operation is reduced.
However, the stopping of the radio transmission may have a large impact on the mobile terminal device and neighboring cells. Thus, when an own cell transfers to an inactive state, specification of a signal notifying the neighboring cell of the transition of the own cell to an inactive state is performed as specific processing for reducing the impact. In addition, a signal requesting an inactive cell to become active is specified (NPTL2). The notification and request messages are usually transmitted via an inter-base-station interface between the base station devices which control target cells.
Furthermore, in a mobile communication system having a plurality of base station devices like a cellular network, mobility control or handover control is performed, which switches base stations so that communication is continued when a mobile terminal device moves from a cell provided one of the base station devices to another cell provided by another of the base station devices. Handover of the mobile terminal device is controlled, based on a value measured and reported by the mobile terminal device, by the base station device providing a cell in which the mobile terminal device is located. Generally, the base station device controls the handover to select a better cell (or best cell) in respect of radio wave reception environment for the mobile terminal device and to hand over the mobile terminal to the selected cell.
Hereinafter, a general handover control procedure is briefly described with reference to FIG. 1. Incidentally, a cell in which a mobile terminal device is present is referred to as a serving cell. A base station device of a serving cell is referred to as a serving base station device. A handover destination cell is referred to as a target cell. A base station device of a target cell is referred to as a target base station device.
In FIG. 1, a serving base station device sets measurement conditions, measurement reporting conditions, and the like by transmitting a measurement setting message M100 to a mobile terminal device. The mobile terminal device measures reference signal received power (RSRP: Reference Signal Received Power), reference signal received quality (RSRQ: Reference Signal Received Quality), or other parameters of each of the serving cell and the neighboring cell according to the measurement conditions set by the serving base station device (operation S100). The parameters includes reference signal received power (RSRP: Reference Signal Received Power), reference signal received quality (RSRQ: Reference Signal Received Quality), or other parameters. Then, if a measurement result satisfies the measurement reporting conditions, a measurement reporting message M101 is transmitted to the serving base station device.
The serving base station device performs handover execution determination, based on a measurement report received from the mobile terminal device (operation S101). In the handover execution determination, a target cell is determined by judging whether handover execution is necessary. In the determination of a target cell, generally, a cell is selected, which is better in radio wave reception environment for the mobile terminal device. Subsequently, the serving base station device transmits, when the target cell is determined, a handover request message M102 including information concerning the mobile terminal device to the target base station device.
The target base station device performs, upon the handover request received from the serving base station device, judging of acceptance of a mobile terminal device (operation S102). The judging of acceptance is performed, based on access control rules such as access authority of the mobile terminal device and a load of the target base station device. If the mobile terminal device is determined to be acceptable, handover preparation such as securement of data resources for the mobile terminal device is executed. Then, if handover is determined to be acceptable, the target base station device transmits, to the serving base station device, a handover request response message M103 including a handover instruction to the mobile terminal device.
Upon the handover request response, the serving base station device transmits, to the mobile terminal device, a handover instruction message M104 received from the target base station device. In response to the handover instruction, the mobile terminal device transmits a handover instruction response to the target base station device. Thus, the handover control procedure is completed.
Incidentally, a method is proposed, which determines, when a target base station is determined, a preferential order by considering not only quality of the radio wave reception environment but capability of the neighboring base station (see PTL1).